Intraprocedural application of a peripheral blood flow monitoring system during endovascular treatment for femoropopliteal disease

Sensors that implement laser speckle image streaming provide real-time, noninvasive assessment of peripheral blood flow during endovascular revascularization. This single-center feasibility study evaluated a laser speckle-based peripheral blood flow monitoring system in 24 patients with peripheral arterial disease. System-quantified blood flow values showed improvement at the conclusion of the procedure in 20 of 24 patients (83.3%). Of the four patients without improved flow values, waveform morphology improved in three. Waveforms graded as moderate to severe peripheral arterial disease decreased from 71% before the procedure to 25% after the procedure, with improvement in 19 of 24 patients. In this limited population, laser speckle imaging could offer a highly sensitive method of detecting intraprocedural pedal blood flow changes.

Approximately 10% of people with peripheral arterial disease have chronic limb threatening ischemia (CLTI), 1 the most severe form of the disease.CLTI is associated with higher amputation and mortality rates compared with less severe disease. 2,3Diagnostic technologies that provide quick and accurate peripheral vascular flow assessments are needed to support optimal management and outcomes.
The current standards of peripheral vascular assessment are the ankle brachial index and toe brachial index (TBI).][10][11][12] Thus, an unmet need exists for a simple, noninvasive diagnostic tool for accurate realtime assessment of vascular flow.
Laser-speckle imaging uses laser light and imaging sensors to measure digital blood flow and assess vascular function.The FlowMet Peripheral Blood Flow Monitoring System (Medtronic) is a laser speckle imaging device in which a sensor is placed on a digit (typically the great toe) and laser light is transmitted from diodes through the tissue. 13A high-speed imaging sensor provides continuous blood flow measurements, displayed as changes in a speckle pattern, which is quantified using a calibrated numeric scale into a flow value and a waveform.The waveform reflects real-time changes in tissue blood flow during the cardiac cycle and enables characterization of blood flow as normal or abnormal, making it particularly amenable to intraprocedural monitoring. 14The waveform differs from a pulse volume recording tracing because it is derived from a direct blood flow measurement, rather than a blood pressure tracing.A prior feasibility study evaluated the laser speckle system in 90 patients and observed that the waveform morphology and amplitude were degraded in the setting of worsening arterial insufficiency and correlated with the TBI. 15

METHODS
This single-center, prospective, all-comers pilot study evaluated the utility of the FlowMet Peripheral Blood Flow Monitoring System (Medtronic) in participants with peripheral arterial disease and planned endovascular revascularization. 15The study was conducted in accordance with the Declaration of Helsinki and approved by the institutional review board at the University of The device was used to monitor tissue blood flow during the planned revascularization.On the procedure day and before generation of a sterile field in the catheterization laboratory, the reusable sensor was clipped onto a treatment limb toe, and the output was recorded using software on the connected tablet computer.The software recorded real-time flow values at a rate of 240 Hz for 30 seconds before the start of the procedure, throughout the revascularization procedure, and for 30 seconds after procedure completion.
The operator was unaware of the flow value and waveform results during the index procedure.Blood flow measurements were quantified using a unitless flow value.Waveforms were categorized using a four-level grading system 16,17 (Fig 1) in a blinded independent review by three of us, with consensus determined by a two-thirds majority vote for each waveform.
No formal power or sample size computations were calculated.A sample size of 24 participants was considered sufficient to summarize the basic descriptive statistics to evaluate the device in this population.Basic descriptive statistics were computed for pre-and postprocedure timepoints and for the waveform change from before to after the procedure, which were categorized as improved, steady, or deteriorated.Continuous variables are summarized as the mean 6 standard deviation and the median with the minimum and maximum values.Categorical variables are summarized as percentages and counts.

RESULTS
Thirty consecutive participants were screened and provided written informed consent.Twenty-four participants (30 limbs) were analyzed.Six participants were excluded because the data were diagnostic only and/or the target lesion could not be accessed or crossed.
Baseline and procedural characteristics.The baseline demographic, clinical, and lesion characteristics are shown in the Table .The mean age of the patients was 66.9 years, and 75% were men.Most (91.7%) presented with Rutherford clinical category $4. Above-the-knee

DISCUSSION
The results from this study demonstrate the feasibility of a laser speckle blood flow monitoring system for use  during peripheral endovascular procedures.The evaluated system generates a flow value and a waveform that closely resembles a pulse volume recording.The waveform depicts the loss of a dicrotic notch and progressive attenuation as the severity of arterial insufficiency increases.As a highly sensitive measure of flow, it demonstrated improvements in both flow value and waveform morphology after successful endovascular intervention.Although the average blood flow values were generally lower for patients with an increasing Rutherford class, deviations from this trend existed and were potentially due to the relatively small sample size.Further evaluation is required to explore how the flow value and waveform improvements seen in this study correlate with other established metrics of disease severity.
Real-time changes in pedal blood flow during peripheral revascularization procedures might allow physicians to monitor improvement, or the lack thereof, during the procedure and react accordingly.In particular, unexpected waveform depression during interventions provides immediate information regarding potential complications compromising flow that might not be otherwise detected until further angiography is performed.As shown previously, the waveform can help in relating system measurements to disease severity by supplementing blood flow measurements that might be altered by covariates such as edema or temperature. 15he ability to obtain this information in real time using a noninvasive system that does not require a large capital expense would be beneficial, in settings of both technical success and unanticipated complications.

CONCLUSIONS
Although these results are promising, this small, singlecenter feasibility study needs to be replicated in larger, higher power studies that more thoroughly gauge performance and outcomes in claudication and CLTI

From
the Division of Interventional Cardiovascular Disease, University of Mississippi Medical Center, Jackson a ; the Division of Vascular Surgery, Sanger Heart & Vascular Institute, Atrium Health, Charlotte b ; and the Division of Vascular Surgery, Iowa Methodist Medical Center, Des Moines.c This study was supported by a collaborative research agreement with Medtronic, which provided assistance with data analysis, figure creation, and manuscript editing.J.G.W. independently collected the study data and had final responsibility for the decision to submit for publication.Data interpretation was the responsibility of the authors.The authors were not paid by Medtronic to conduct this study or write this article.Presented as a late-breaking trial at the Twelfth Annual Amputation Prevention Symposium, Chicago IL, August 17-20, 2022.Correspondence: John G. Winscott, MD, Department of Interventional Cardiovascular Disease, University of Mississippi Medical Center, 2500 N State St, Jackson, MS 39216 (e-mail: jwinscott@umc.edu).The editors and reviewers of this article have no relevant financial relationships to disclose per the Journal policy that requires reviewers to decline review of any manuscript for which they may have a conflict of interest.2468-4287 Ó 2023 The Authors.Published by Elsevier Inc. on behalf of Society for Vascular Surgery.This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).https://doi.org/10.1016/j.jvscit.2023.101369Mississippi Medical Center (Jackson, MS).All the patients provided written informed consent before enrollment.Eligible patients were $18 years of age with infrainguinal disease and/or CLTI and a clinical diagnosis of peripheral vascular disease (Rutherford clinical class 2-6) requiring endovascular revascularization.Data were obtained from the medical records of diagnostic testing performed before, during, and immediately after revascularization.

Fig 1 .
Fig 1. Four-level grading system for waveform characterization.PAD, Peripheral arterial disease.Defined by Rumwell and McPharlin 16 in 1998, and reproduced, with permission, from Lewis et al, 17 under the terms of a Creative Commons Attribution-NonCommercial 3.0 License.

Fig 2 .
Fig 2. Flow values before and after the endovascular procedure.A, Flow values in all 24 patients.Bars from top to bottom are maximum, quartile 3, median, quartile 1, and minimum.The mean is shown with an X. B, Mean follow values before and after the procedure stratified by baseline Rutherford clinical class (RCC).

Fig 3 .
Fig 3. Flow values and morphology before and after the endovascular procedure.A, Individual participants shown using a log-2 scale before and after the revascularization procedure.B, Waveform morphologies before and after the endovascular procedure.

Fig 4 .
Fig 4. A, A generic example of the FlowMet Peripheral Blood Flow Monitoring System screen (Medtronic).B, An example waveform before the procedure.C, An example waveform after the procedure.